Ruby on Rails: Understanding, Optimizing, and Mastering Database Queries with Preloading, Eager Loading, Includes, and Joins

Hi, This is Shubham Mishra from India,  this is the part of Ruby on Rails exploration journey.  In this post we will discuss about the ruby's some special concepts and I am sure you will get excited to make your hands dirty with those code, so why to wait let's get started...

In this article we will explore best practices to avoid N+1 query issue in ruby on rails and also best practices to make database query optimised. And we also need to keep below stuffs in mind while doing the code review of our peer.

Ruby on Rails offers a number of methods for preloading related items in order to get around the N+1 query problem. When the application does his N+1 database queries, the N+1 query problem arises. where N represents the total number of records found. Serious performance problems may result from this, especially when working with huge datasets.
Using eager loading, which preloads related objects when the database is requested, is one way to deal with this issue. An includes method that specifies the associations to preload enables eager loading. for instance:


In the above example, the first code snippet generates N+1 queries, where each call to post.comments.count triggers a new query. The second code snippet uses eager loading to preload the associated comments, resulting in only two queries being executed.

Another technique for preloading associated objects is to use the joins method to join tables and retrieve the necessary data in a single query. This can be particularly useful when querying multiple associations at once. For example:


In the above example, the first code snippet generates N+2 queries, where each call to user.posts.count and user.comments.count triggers a new query. The second code snippet uses a join to retrieve all necessary data in a single query, resulting in only one query being executed. In addition to eager loading and joins, Ruby on Rails also provides other techniques for preloading associated objects, such as preload, eager_load, and includes(:association).references(:association). The choice of technique will depend on the specific use case and performance requirements of the application.

Here is an example of how preloading associated objects can improve performance:


In the above example, the first code snippet generates N+1 queries for each call to post.comments.each, resulting in a significant performance hit. The second code snippet uses eager loading to preload the associated posts and comments, resulting in only two queries being executed and a much faster execution time. In summary, preloading associated objects is an important technique for improving the performance of Ruby on Rails applications. Eager loading, joins, and other preloading techniques can help to reduce the number of database queries and improve application response times.

What are the alternative to preloading and also give example, advantage and conditions to use for all alternatives?

Apart from preloading associated objects, there are a few alternatives available in Ruby on Rails to optimize database queries and improve application performance. Here are some of them:

• Caching: Caching can be used to store frequently accessed data in memory or on disk, which can reduce the number of queries required to retrieve data. Ruby on Rails provides caching support through its Rails.cache API and various caching strategies such as fragment caching, action caching, and page caching.
  Example:
  
  
  In the above example, the first code snippet retrieves all posts from the database on each request. The second code snippet caches the posts for five minutes and retrieves them from the cache on subsequent requests, which can significantly improve performance.
  Advantage: Caching can help to reduce database load and improve application response times.
  Conditions to use: Caching is best suited for data that doesn't change frequently and doesn't need to be up-to-date at all times.


• Database Indexing: Database indexing can be used to speed up database queries by creating an index on one or more columns. This can improve the speed of queries that use those columns in a WHERE clause or an ORDER BY clause.
  Example:
  
  
  In the above example, an index is created on the user_id column, which can improve the speed of queries that join the posts and users tables.
  Advantage: Database indexing can improve the performance of queries that use the indexed columns.
  Conditions to use: Database indexing is best suited for large tables with frequent queries that use the indexed columns.


• Query optimization: Query optimization involves optimizing database queries to reduce their execution time. This can be achieved by using techniques such as query caching, using efficient SQL queries, and avoiding unnecessary calculations.
  Example:
  
  
  In the above example, the first code snippet retrieves all posts from the database and orders them by created_at. The second code snippet only retrieves the necessary columns and includes the associated user, which can improve query performance.
  Advantage: Query optimization can improve the performance of database queries.
  Conditions to use: Query optimization is best suited for complex queries and large tables with slow query times.

preloading associated objects is not the only technique available in Ruby on Rails to optimize database queries and improve application performance. Caching, database indexing, and query optimization are other techniques that can be used in conjunction with preloading to achieve optimal performance. The choice of technique will depend on the specific use case and performance requirements of the application

Now Now check the difference between preload vs eager loading vs includes vs joins

In Ruby on Rails, preload, eager loading, includes, and joins are all techniques used to fetch associated objects from the database. However, there are some important differences between them:

1. preload: preload is a method that fetches the associated objects in a separate query. It is used to avoid the N+1 query problem where N queries are executed to fetch N associated objects. With preload, only two queries are executed - one to fetch the primary objects and another to fetch the associated objects.
   Example:
   
   
   In the above example, the first code snippet will execute N+1 queries to fetch the associated users. The second code snippet will execute only two queries - one to fetch the posts and another to fetch the associated users.
   
2. eager loading: eager loading is similar to preload, but it fetches the associated objects in a single query using a JOIN statement. It is used when preload is not sufficient and multiple associated objects need to be fetched.
   Example:
   
   
   In the above example, the first code snippet will execute N+1 queries to fetch the associated comments. The second code snippet will execute a single query with a JOIN statement to fetch both the posts and comments.
   
3. includes: includes is a method that can be used with both preload and eager loading to specify the associated objects to fetch. It is used to reduce the number of queries executed when fetching associated objects.
   Example:
   
   
   In the above example, the includes method is used to fetch the associated user object and the preload method is used to fetch the associated comments object.
   
4. joins: joins is a method that can be used to fetch records from multiple tables based on a common attribute. It is used to execute a single SQL query that joins multiple tables and fetches the required data.
   Example:
   
   
   In the above example, the joins method is used to join the posts and users tables based on the user_id attribute and fetch both the post and user data in a single query.
   

In summary, the differences between preload, eager loading, includes, and joins are as follows:
preload: Fetches associated objects in a separate query
eager loading: Fetches associated objects in a single query using a JOIN statement
includes: Used with both preload and eager loading to specify the associated objects to fetch
joins: Used to fetch records from multiple tables based on a common attribute in

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